This invention relates generally to control circuits for electric motors and more particularly to an improved electronic starting control circuit for a multi-speed alternating current motor.
Single phase alternating current motors normally include a start winding in addition to one or more run windings. The start winding and run winding are generally physically displaced from each other on the stator of the motor and carry currents that are displaced in phase. To start such motors, both windings are connected to a single phase alternating current supply line and, because of the phase displacement between their currents, starting torque is developed. As the motor approaches operating speed, it is known that it is desirable to disconnect the start winding from the supply line in order to operate the motor more efficiently. Additionally, since most start windings are designed to be energized for only very short periods of time, they are not designed to carry current for longer periods of time, and failure to disconnect the start winding from the alternating current source would probably result in serious damage to the start winding. Heretofore, the disconnection of the start winding from the alternating current source has commonly been accomplished by use of a mechanical centrifugal switch in the start winding circuit. Such mechanical switches are relatively inexpensive devices but have a life span which is usually substantially shorter than the life span of various electronic switching devices. In order to take advantage of the longer life span of electronic switching devices, voltage and current relays including solid state switches have been utilized to disconnect the start winding from the alternating current source. However, this approach to disconnecting motor start windings has the disadvantage that for different size motors, different circuits must be utilized. As a result, this solution has proved to be uneconomical.
A novel solution to the problem of electronically disconnecting the start winding is disclosed and claimed in the referenced copending application. A starting control circuit is provided with an alternating current motor having a run winding and a start winding and a mechanical rotational output. The motor is powered by an alternating current source. A reference oscillator produces pulses having a reference frequency. A first control circuit is provided for counting the number of reference pulses produced during a predetermined number of cycles of the alternating current source. A sensor is provided for sensing a predetermined amount of rotational movement of the mechanical rotational output of the motor and producing a sensor pulse indicative thereof. A second control circuit is provided for counting the number of reference pulses produced during the time between the predetermined number of sensor pulses and for comparing the counts on the first and second control circuits. The second control circuit assumes first and second output states dependent on the relationship between the first and second output counts. A switch is connected to the second control circuit, the alternating current source and the start winding for connecting and disconnecting the alternating current source and the start winding in response to the output state of the second control circuit.
The present invention uniquely adapts the invention of the referenced copending application for use with multispeed motors and specifically for use with a single phase, two-speed induction motor. These two-speed motors are normally provided with two run windings and two start windings and operate in a manner similar to the above-described single-speed motors. When one set of windings is energized, the motor will run close to the synchronous speed for the pole and supply frequency combination of that winding. Similarly, the other set of windings produce a different synchronous speed. The present invention allows the features of the referenced copending application to be used with two-speed induction motors.